The present invention relates to a mast apparatus for lift trucks and a lift truck equipped with a mast apparatus. More specifically, the invention relates to a mast apparatus and a lift truck, in which a carrier for carrying a load-handling attachment is lifted and lowered by a lift chain along a guide mast, and the lift chain tends to swing.
FIGS. 1 and 2 show typical states of a conventional mast apparatus in the form of a mast-guided fork lift, with a fork lowered to and slightly lifted from a floor or ground. The mast apparatus has a pair of left and right outer rails 1, in a view along a longitudinal direction of a vehicle body (not shown) of a fork lift truck equipped with the mast apparatus. The left and right outer rails 1 are interconnected for integration by a plurality of beam members extending in a transverse direction of the vehicle body. A lowest beam member 3L has support brackets 5 integrally provided thereto for a pivotal support on a front axle (not shown) as part of a body of the fork lift truck to permit a pivotal motion in the longitudinal direction. An upper beam member 3U is provided with L-shaped brackets 4 at left and right ends thereof, and is integrally connected via the brackets 4 to the left and right outer rails 1. For the outer rails 1 to be longitudinally tilted relative to the vehicle body, left and right tilt cylinders 9 are interposed therebetween. Each tilt cylinder 9 is pivoted at a base end thereof on a part 7 of the vehicle body, and at an end of a piston rod 9P thereof to one of the outer rails 1.
The left and right outer rails 1 have a pair of left and right inner rails 11 provided on transversely inner sides thereof in a vertically movable manner. The left and right inner rails 11 are interconnected for integration by a plurality of beam members extending in the transverse direction. An uppermost beam member 13U projects at left and right ends thereof outside the left and right outer rails 1. The left and right ends of the uppermost beam member 13U each have a chain wheel bracket 15 integrally provided thereto.
A chain wheel 17 is provided on a transversely inner side of the chain wheel bracket 15, and is rotatably supported thereon. A lift chain 19 is applied over the chain wheel 17, and is connected at one end thereof to an anchor bolt 23, which is provided to a lower part of a carriage 21 that is vertically movable along the inner rails 11 that serve as guide masts therefor. The other end of the lift chain 19 is connected to another anchor bolt 25, which is attached near an upper part of outer rail 1 in a vertically shiftable manner. The carriage 21 carries a plurality of forks 27 for handling a load.
The inner rails 11 are actuated to vertically move relative to the outer rails 1, so that the carriage 21 and the forks 27 are lifted and lowered along the inner rails 11. For the actuation, the outer rails 1 are equipped with a pair of left and right lift cylinders 29, which have their vertically movable piston rods 29P connected to the uppermost beam member 13U between the inner rails 11.
As the piston rods 29P of the lift cylinders 29 vertically move, the inner rails 11 vertically move relative to the outer rails 1, causing the carriage 21 and the forks 27 to be lifted and lowered. When the fork lift truck travels, the forks 27 are held in a slightly lifted position as in FIG. 2.
As the fork lift truck travels, a length of the lift chain 19 between the chain wheel 17 and the anchor bolt 23 has a varying tendency to swing in the longitudinal and transverse directions, as in FIG. 2, depending such as on the condition of travel route as well as on a loading condition to the forks 27. The lift chain 19 may have a large swing, directly hitting the outer rail or the like, causing a coat on the chain such as grease to be spattered, producing uncomfortable sounds, and/or interrupting a sight of a lift operator. Frequent direct hits on the outer rail may render a potential chain life shorter.